This invention is directed to a booster circuit for use in a small-sized battery operated electronic instrument, such as an electronic wristwatch, and in particular to a booster circuit that is capable of producing a predetermined output voltage by consuming additional current when the load driven by the output voltage of the booster circuit varies.
In miniaturized electronic instruments, such as electronic wristwatches, a DC battery is utilized as the only voltage supply. Accordingly, the battery should have a large capacity yet be small in size. To this end, miniaturized silver batteries, or mercury batteries, are utilized in most electronic wristwatches now being manufactured. Nevertheless, one disadvantage of utilizing silver or mercury batteries is the low voltage delivered thereby, thereby rendering it necessary to utilize more than one battery or, alternatively, to utilize a booster circuit in order to drive the electronic timepiece circuitry. Although it is easy to increase the voltage delivered by the power supply by increasing the number of batteries, the increased number of batteries requires a corresponding increase in the size of the wristwatch, and accordingly from a standpoint of miniaturization, a booster circuit is preferred.
Nevertheless, in order to effectively utilize a booster circuit, it is important that the current consumption of the booster circuit be minimized in order to prevent the battery from being unnecessarily dissipated thereby. It is noted, however, that the reduced current consumption obtained by a booster circuit must be balanced against the necessity of producing a predetermined output voltage for driving the electronic wristwatch when a maximum load is placed upon the voltage supply for a short interval of time. Such sudden increases in the power required to operate an electronic wristwatch are a result of the improved integrated circuit techniques and electro-optical display techniques that have, in recent years, permitted miniaturized electronic wristwatches to function as a stopwatch, a calculator, etc., without any loss in accuracy or size. Nevertheless, when an electronic wristwatch performs an additional function, a considerable load is placed upon the voltage supply, thereby causing the booster circuit that had heretofore been designed for minimum current consumption to be unable to produce a sufficient output voltage for driving the additional load placed upon the voltage supply. Accordingly, a booster circuit wherein the current consumed is varied, in response to a change in the load driven by the output voltage produced by the booster circuit, is desired.